In many applications, it is necessary to deposit thin layers on a substrate, e.g. a flexible substrate. Typically, the flexible substrates are coated in different chambers of a flexible substrate coating apparatus. Further, a stock of a flexible substrate, for example, a roll of a flexible substrate, may be disposed in one chamber of the substrate coating apparatus. Typically, the flexible substrates are coated in a vacuum, using a vapor deposition technique, for example, physical vapor deposition or chemical vapor deposition.
Particularly for flexible substrates but also for other substrates, the deposition rate of a deposition process is of relevance, because deposition on flexible substrates is often conducted whilst the flexible substrate is fast moving. For example, in packaging industry, layers like Al2O3, SiO2 or other layers, which can be deposited by a reactive deposition process, can be used for coating packaging.
For example, SiO2 can be sputtered in oxygen mode but in this mode the deposition rate is low. In metallic mode an absorbing SiOx layer is deposited. Therefore, it is most efficient to run the SiO2 process in transition mode. In this mode it is possible to deposit transparent SiO2 with high rate. To keep the cathode in the transition mode special monitoring features like PEM (plasma emission monitoring) or lambda sensors can be used in combination with a special control routine. This needs extra hard- and software and is therefore an expensive solution. Accordingly, a fast and reliable reactive deposition process is required, which is simpler to implement.